Ultrahigh purity chemicals are used as source chemicals in Chemical Vapor Deposition (CVD) processes. Three methods currently are utilized for delivering the chemical to the process chamber. One of these methods is known as direct liquid injection.
Direct liquid injections systems typically employ liquid (or mass) flow control systems that are sensitive to the presence of dissolved gases in the liquid CVD source chemical. These liquid flow controllers are necessary to deliver precise quantities of the source chemical to the process tool. The sensitivity of these devices is a result of the thermodynamic operating principal of typical liquid flow controllers and can be better understood by a description of the operation of such a device.
A typical liquid flow controller has a precision power supply that directs heat to the midpoint of a sensor tube which is carrying a constant percentage of the flow to be measured. On the same sensor tube, temperature measuring elements are placed equidistant upstream and downstream of the heat input. If no liquid or gas is flowing in the sensor tube, the heat reaching each temperature element is equal. However, with increasing flow, the flow stream carries heat away from the upstream element toward the downstream element. An increasing temperature difference develops between the two temperature measuring elements and this temperature difference is proportional to the liquid or gas flow rate.
If the liquid flow being measured is devoid of dissolved gases, a thermodynamic mass flow controller can accurately detect the amount of liquid flowing through the sensor tube. For typical CVD systems, mass flow is monitored in the vapor phase and is not sensitive to dissolved gases. However, in direct liquid injection applications, inert gases, such as helium and nitrogen, are used to pressurize CVD chemical source canisters to force the chemical contents out of the container to the process tool. Therefore, dissolved gases may be present in chemicals dispensed under pressure from ultrahigh chemical containers.
As a result of this thermodynamic operating principle, liquid flow controllers are very sensitive to gas bubbles being present in the liquid stream, because the heat capacity of the inert gas is considerably less than the liquid being transferred. If gas bubbles are, in fact, present in the liquid stream, the liquid flow controller will experience a period of instability in which improper amounts of liquid are delivered to the process tool. If this instability occurs, film deposition non-uniformities on the IC being manufactured can result and may lead to the fabrication of defective ICs. Therefore, it would be desirable to design a direct liquid injection system that minimizes dissolved gases in the liquid stream.
Novellus Systems, the first CVD process tool manufacturer to adopt direct liquid injection for tetraethylorthosilicate (TEOS) based CVD processes, dealt with the dissolved helium pressurizing gas problem by periodically "degassing" the source chemical container. The degassing procedure consists of evacuating the helium pressurizing gas from the source container and maintaining a vacuum in the container for a period of time to remove dissolved helium from the source chemical. However, this procedure adds additional degassing requirements and process delays while the liquid is being degassed. To avoid equipment downtime that results from a degassing step, Novellus also has developed an in-line degassing apparatus that eliminates the need to pull a vacuum on the source chemical container. Thus, this system requires an additional degassing module.
More recently, Applied Materials, another CVD process tool manufacturer, imposed a minimum distance from the surface of the liquid chemical to the bottom of the dip tube in a 5 gallon CVD chemical source container, because the process tool developed flow instability when the chemical level went lower. It was concluded that a concentration gradient of dissolved helium pressurizing gas would create more serious flow instability problems as the gas liquid interface approached the bottom of the container. The minimum distance established in the chemical container was at a level equivalent to 40% of the container volume.
Many process tools require the use of triethyl phosphate (TEPO), triethyl borate (TEB) and other ultrahigh purity chemicals in very low volume. The delivery conditions described above create unacceptable conditions for the five gallon containers traditionally used, as all of the chemical may not be consumed before the shelf life of the chemical is exceeded.